1. Field of the Invention
The present invention relates generally to integrated circuits and, more particularly, to integrated circuits implementing transistors that have been forward biased between the source region and the body to increase the depth of the induced channel.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As most people are aware, an integrated circuit is a highly miniaturized electronic circuit that is typically designed on a semiconductive substrate. Over the last 10 years, there has been considerable attention paid to designing smaller, lower-power integrated circuits. These smaller, lower-power integrated circuits are often used in portable electronic devices that rely on battery power, such as cellular phones and laptop computers. As circuit designers research new ways to lower the power consumption of integrated circuits, they are constantly confronted with new challenges that need to be overcome in order to create the integrated circuits that will be part of the next generation computer, cellular phone, or camera.
The fundamental building block of the modern integrated circuit is the transistor. Transistors are most often created on a substrate composed of a silicon semiconductor, but they may be created using any one of a number of different semiconductors. Silicon transistors are created by altering the electrical properties of silicon by adding other materials called “dopants” to the silicon. This process is known as doping. In n-type doping, dopants are added to the silicon to provide extra electrons that do not bond with the silicon. These free electrons make n-type silicon an excellent conductor. In p-type doping, silicon is doped with elements that cause an empty space, known as a “hole,” to develop in the silicon. Because these holes readily accept electrons from other silicon atoms, p-type silicon is typically also a good conductor.
Even though p-type silicon and n-type silicon are each good conductors, they are not always good conductors when joined. These junctions, called p-n junctions, are essential one way streets for current—allowing it to flow in one direction across the junction but not in the other direction. When current can flow across the p-n junction, it is said to be “forward-biased,” and when current can not flow across the p-n junction, it is considered to be “reverse-biased.”
A transistor is created by combining two p-n junctions. For example, a transistor might be arranged as either NPN or PNP. In this arrangement, a relatively small current (or voltage, depending on the type of transistor) applied to the center layer will essentially “open up” the transistor and permit a much greater current to flow across the transistor as a whole. In this fashion, transistors can act as switches or as amplifiers.
While there are numerous types of transistors, metal-oxide semiconductor field-effect transistors (“MOSFETs”) have been particularly popular over the past few years. One example of this type of MOSFET is known as an n-channel enhancement type MOSFET or NMOS transistor. The NMOS transistor is created by forming two heavily doped n-type regions in a p-type semiconductive substrate (i.e. NPN). These two n-type regions form regions known as the source and drain regions. Next, a thin layer of an oxide insulator may be grown on the surface of the substrate and metal or another conductor may be deposited on this oxide to create a gate region. Terminals are then attached to the source region, the drain region, the gate region, and the p-type semi-conductive substrate (also known as “the body”) to create a semiconductor device with four terminals: the source (“S”) terminal, the drain (“D”) terminal, the gate (“G”) terminal, and the body (“B”) terminal.
A voltage Vgs placed between the gate terminal and the source terminal of the NMOS transistor will create an electrical field in the semiconductive substrate below the gate terminal. This electrical field causes mobile electrons in the source region, the drain region, and the substrate to accumulate and form an n-type conductive channel in the p-type substrate. This conductive channel is known as the “induced channel.” This n-type induced channel effectively connects the source and drain regions together and allows current to flow from the drain to the source (i.e. opening up the transistor). The voltage Vgs that is sufficient to cause enough electrons to accumulate in the channel to form an induced channel (i.e. to open up the channel) is known as the “threshold voltage.”
A related type of MOSFET, known as p-channel enhancement type MOSFET or PMOS, is created on an n-type substrate with source and drain regions composed of p-type regions (i.e. PNP). PMOS transistors operate very similarly to NMOS transistors except that the threshold voltage is negative and current flows from the source terminal to the drain terminal.
As stated above, MOSFETs have four terminals: the source, the drain, the gate, and the body. Of these, the body terminal is the least well-known. This is the case because in most early applications, the body terminal was electrically coupled to the source terminal. Connecting the source and body regions together creates a constant reverse bias on the p-n junction between the body and the channel. Because current can not flow across this reverse biased p-n junction, no current could flow into the body, and thus the body typically did not affect the operation of the transistor.
Unfortunately, this same concept does not always apply when there are multiple integrated circuits sharing a single body as with an integrated circuit. Because there are many transistors connected to the same body, it is no longer certain that connecting the source region to the body will create a constant reverse bias. One method of ensuring that the reverse bias is maintained is to connect the body to the most negative power supply in the NMOS MOSFET or the most positive power supply in a PMOS MOSFET. However, this large reverse bias can reduce the depth of the induced channel. Disadvantageously, as the channel becomes shallower, the amount of current that can flow through the induced channel is reduced even though the voltage Vgs stays constant. This phenomenon is known as the “body effect.” In order to counter the body effect, the voltage Vgs may be increased. Years ago when power consumption was not a top priority for circuit designers, increasing Vgs did not present a serious problem. In recent years, however, with the rapid growth of mobile technologies that rely on battery power, scientists and engineers have searched for a way to maintain or increase the induced channel current (i.e. deepen the induced channel current) without increasing the voltage Vgs.
One recent method to increase the channel current without increasing the voltage Vgs is to forward bias the p-n junction between the source terminal and the body terminal. In accordance with these techniques, the PMOS body is usually biased lower than the source terminal voltage and the NMOS body is usually biased higher than the source terminal voltage. The forward biasing increases the channel depth, which permits the transistor to conduct more current for a given voltage Vgs. A channel that can conduct more current can be used to make the transistor operate faster on the same voltage or to reduce the size of the transistor without sacrificing performance.
Unfortunately, forward-biasing the source to body p-n junction can have unintended effects on the circuit. Foremost amongst these effects is the potential for body leakage current. As previously discussed, when a p-n junction is forward biased, it is essentially opened up to the flow of current. This may permit the current flowing across the channel to “leak” into the body of the transistor. Because this leakage current reduces the amount of current that flows between the source and the drain, it can have an adverse affect on the performance of the transistor. This is especially the case if the transistor is driving a particularly small load, and the current across the transistor is relatively small. For larger currents, some leakage current may be permitted to maximize the potential induced channel current, but if this leakage current is not limited in some fashion, its effects can overshadow the potential increase in induced channel current. A circuit that can minimize leakage current for small loads and clamp leakage current for higher loads is desirable.
Embodiments of the present invention may address one or more of the problems set forth above.